There is a vital need to understand the causes of depression in order to develop effective treatments for the 9% of Americans who currently suffer from this debilitating disease. This project focuses on a new target, T helper 17 (Th17) cells, which we recently linked to depression susceptibility in mouse models and for which we identified feasible interventions. The overall objectives of this project are to identify mechanism regulating Th17 cell production and test the potential therapeutic impact of targeting Th17 cells to decrease vulnerability to depression, assessed by measuring depression-like behaviors in mice. This research evolved from the now well-established link between inflammation and depression. We reasoned that therapeutically targeting downstream, and prolonged, outcomes of inflammation may be more feasible than attempting to neutralize the multitude of cytokines that are transiently induced in the inflammatory response to stress. Inflammatory cytokines associated with depression drive the production of Th17 cells, and Th17 cells are already well-established to be toxic to the CNS. In mouse models, we found that Th17 cells accumulate in the brain during induced depression-like states, that administration of Th17 cells was sufficient to increase vulnerability to depression-like outcomes and that ablating the production or actions of Th17 cells provided strong resistance to depression-like outcomes of stress. Thus, it is now critical to identify mechanisms that control the production and effects of Th17 cells to obstruct their pro-depressant actions. Remarkably, we found that glycogen synthase kinase-3 (GSK3), an enzyme already known to promote depression susceptibility, drives the production of Th17 cells. Inhibition of GSK3 blocked Th17 cell production in vitro and in vivo in a variety of conditions, and reduced susceptibility to stress-induced depression-like outcomes in mice. In this project, Specific Aim 1 will determine the mechanisms by which GSK3 controls Th17 cell production during depression. We will identify transcription factors involved in Th17 cell differentiation regulated by GSK3, and regulating GSK3 induced expression in these cells, and identify characteristics of depression-induced Th17 cells. Specific Aim 2 will test if targeting subtypes of Th17 cells increases resistance and recovery of depression-like behavior in mice. Using nanoparticle-coupled drugs and GSK3 siRNA, and transgenic mice with GSK3-depleted T cells, we will identify new interventions mechanisms with minimal side effects. Specific Aim 3 will test the hypothesis that IL-17A that is produced by Th17 cells mediates increased susceptibility to depression-like behaviors in mice. Altogether this project will identify the mechanisms how Th17 cells promotes depression-like outcomes and determine mechanisms by which Th17 cell production following stress can be blocked in order to develop a new therapeutic strategy for depression, a prevalent, debilitating, and inadequately treated disease.